Mojotone 18 TMB + No Sound = Sadness
Posted: Thu 01/04/24 5:51 pm
First time poster, but I sure do appreciate all of the knowledge that gets shared on here. I've been lurking for a while and I've learned a lot, and only wish that I had read more before starting this build.
I'm working on a Mojotone 18 watt TMB kit; had I known the collective wisdom before I started, I might have gone a different route. I had an easy time with a Weber Princeton kit, but this one is giving me some fits. I'm not getting any sound out of the speaker at all -- not a hum, hiss, or scratch. The cables are good and the speaker is good, so I don't think those are causing any issues.
I went through Rob Robinette's start-up guide and my light bulb limiter dims until I'm at the power tube step -- I'd like to believe that the bulb dims, but I'm not convinced that it actually does. At this point, I've traced the schematic a few times, confirmed connections and components, but I'm not seeing anything that jumps out at me. I've reflowed all of the solder connections and I can absolutely confirm that the jumpers noted on the diagram are there, but on the back of the board. I know that older tubes don't have the 1, 6, and 8 IC pins that the layout would use, but the EHX tube sheet notes that they are "free".
You'll notice that I've got that yellow/white wire from the PT floating towards the ground terminal. The layout had it connected to pin 6 of EZ81, which is how I did it at first; however, that is not what the schematic shows, so I have it disconnected and will insulate it once I've got things working. It looks like it is an IC, though, so I'm not sure how much it matters.
I've attached the schematic and the newest version of the Mojotone layout, in addition to some images.
Here are my measured voltages, with Mojotone's specified voltages in parantheses (given the nature of the amp, they are higher than the voltages listed on the document from the Downloads page).
V1
1: 163 (151)
3: 1.1 (1)
6: 225 (190)
8: 2.0 (1.5)
V2
1: 165 (231)
3: 1.1 (2.5)
6: 169 (148)
8: 1.1 (1)
V3
1: 246 (213)
3: 86 (68)
6: 260 (213)
8: 86 (68)
V4
3: 31 (12)
9: 404 (330)
V5
3: 31 (12)
9: 404 (330)
I've obviously got problems somewhere!
The following are things that I know that I need to do:
- use shielded wire for the inputs
- move those 68K RFI resistors closer to V1
- mount the cap can to a better spot
Once I get an actual signal, I'll work on making it better.
Any help is most appreciated!
I'm working on a Mojotone 18 watt TMB kit; had I known the collective wisdom before I started, I might have gone a different route. I had an easy time with a Weber Princeton kit, but this one is giving me some fits. I'm not getting any sound out of the speaker at all -- not a hum, hiss, or scratch. The cables are good and the speaker is good, so I don't think those are causing any issues.
I went through Rob Robinette's start-up guide and my light bulb limiter dims until I'm at the power tube step -- I'd like to believe that the bulb dims, but I'm not convinced that it actually does. At this point, I've traced the schematic a few times, confirmed connections and components, but I'm not seeing anything that jumps out at me. I've reflowed all of the solder connections and I can absolutely confirm that the jumpers noted on the diagram are there, but on the back of the board. I know that older tubes don't have the 1, 6, and 8 IC pins that the layout would use, but the EHX tube sheet notes that they are "free".
You'll notice that I've got that yellow/white wire from the PT floating towards the ground terminal. The layout had it connected to pin 6 of EZ81, which is how I did it at first; however, that is not what the schematic shows, so I have it disconnected and will insulate it once I've got things working. It looks like it is an IC, though, so I'm not sure how much it matters.
I've attached the schematic and the newest version of the Mojotone layout, in addition to some images.
Here are my measured voltages, with Mojotone's specified voltages in parantheses (given the nature of the amp, they are higher than the voltages listed on the document from the Downloads page).
V1
1: 163 (151)
3: 1.1 (1)
6: 225 (190)
8: 2.0 (1.5)
V2
1: 165 (231)
3: 1.1 (2.5)
6: 169 (148)
8: 1.1 (1)
V3
1: 246 (213)
3: 86 (68)
6: 260 (213)
8: 86 (68)
V4
3: 31 (12)
9: 404 (330)
V5
3: 31 (12)
9: 404 (330)
I've obviously got problems somewhere!
The following are things that I know that I need to do:
- use shielded wire for the inputs
- move those 68K RFI resistors closer to V1
- mount the cap can to a better spot
Once I get an actual signal, I'll work on making it better.
Any help is most appreciated!